Energy is everywhere in the environment and it is derivable from several natural sources in the form of thermal energy, solar energy, wind energy, and mechanical/kinetic energy. Energy harvesting is a process in which energy is derived from external sources, captured and stored, for example, for small, wireless autonomous i.e. self-powered devices, such as a variety of sensor and control circuitry, or energy harvesting can be used as an alternative energy source to supplement primary power and to enhance the reliability of the overall system and to prevent power interruptions.
When energy harvesting devices i.e. devices using energy which is captured from the environment and transformed into electrical energy, use energy harvesting as an only energy source it is possible that there are times when energy is not available or that the used energy source does not produce enough energy for powering the energy harvesting device. These conditions can cause problems for energy harvesting devices or systems using the energy harvesting devices.